make PyTypeCheck an unsafe trait

guide/src/migration.md

@@ -57,7 +57,7 @@ Before:
 struct MyWrapper<T>(T);
 
 impl<'py, T> FromPyObject<'py> for MyWrapper<T>
-where 
+where
     T: FromPyObject<'py>
 {
     fn extract_bound(obj: &Bound<'py, PyAny>) -> PyResult<Self> {
@@ -66,7 +66,7 @@ where
 }
 ```
 
-After: 
+After:
 ```rust
 # use pyo3::prelude::*;
 # #[allow(dead_code)]
@@ -133,6 +133,12 @@ This is very similar to `serde`s [`Deserialize`] and [`DeserializeOwned`] traits
 [`DeserializeOwned`]: https://docs.rs/serde/latest/serde/de/trait.DeserializeOwned.html
 </details>
 
+## `PyTypeCheck` is now an `unsafe trait`
+
+Because `PyTypeCheck` is the trait used to guard the `.cast()` functions to treat Python objects as specific concrete types, the trait is `unsafe` to implement.
+
+This should always have been the case, it was an unfortunate omission from its original implementation which is being corrected in this release.
+
 ## from 0.25.* to 0.26
 ### Rename of `Python::with_gil`, `Python::allow_threads`, and `pyo3::prepare_freethreaded_python`
 <details open>

newsfragments/5473.changed.md

@@ -0,0 +1 @@
+Make `PyTypeCheck` an `unsafe trait`.

src/type_object.rs

@@ -35,6 +35,10 @@ pub trait PySizedLayout<T>: PyLayout<T> + Sized {}
 ///
 /// Implementations must provide an implementation for `type_object_raw` which infallibly produces a
 /// non-null pointer to the corresponding Python type object.
+///
+/// `is_type_of` must only return true for objects which can safely be treated as instances of `Self`.
+///
+/// `is_exact_type_of` must only return true for objects whose type is exactly `Self`.
 pub unsafe trait PyTypeInfo: Sized {
     /// Class name.
     const NAME: &'static str;
@@ -85,7 +89,13 @@ pub unsafe trait PyTypeInfo: Sized {
 }
 
 /// Implemented by types which can be used as a concrete Python type inside `Py<T>` smart pointers.
-pub trait PyTypeCheck {
+///
+/// # Safety
+///
+/// This trait is used to determine whether [`Bound::cast`] and similar functions can safely cast
+/// to a concrete type. The implementor is responsible for ensuring that `type_check` only returns
+/// true for objects which can safely be treated as Python instances of `Self`.
+pub unsafe trait PyTypeCheck {
     /// Name of self. This is used in error messages, for example.
     #[deprecated(
         since = "0.27.0",
@@ -108,7 +118,7 @@ pub trait PyTypeCheck {
     fn classinfo_object(py: Python<'_>) -> Bound<'_, PyAny>;
 }
 
-impl<T> PyTypeCheck for T
+unsafe impl<T> PyTypeCheck for T
 where
     T: PyTypeInfo,
 {

src/types/weakref/anyref.rs

@@ -18,7 +18,7 @@ pyobject_native_type_named!(PyWeakref);
 // #[cfg(not(Py_LIMITED_API))]
 // pyobject_native_type_sized!(PyWeakref, ffi::PyWeakReference);
 
-impl PyTypeCheck for PyWeakref {
+unsafe impl PyTypeCheck for PyWeakref {
     const NAME: &'static str = "weakref";
 
     #[cfg(feature = "experimental-inspect")]

src/types/weakref/proxy.rs

@@ -20,7 +20,7 @@ pyobject_native_type_named!(PyWeakrefProxy);
 // #[cfg(not(Py_LIMITED_API))]
 // pyobject_native_type_sized!(PyWeakrefProxy, ffi::PyWeakReference);
 
-impl PyTypeCheck for PyWeakrefProxy {
+unsafe impl PyTypeCheck for PyWeakrefProxy {
     const NAME: &'static str = "weakref.ProxyTypes";
 
     #[cfg(feature = "experimental-inspect")]